Synchronize on mHandler in injectPostToHandlerDebounced

This change updates injectPostToHandlerDebounced to synchronize on
mHandler instead of mServiceLock to fix the contention seen in
b/394973597.

Bug: 394973597
Test: atest CtsShortcutManagerTestCases ShortcutManagerTest{1..11}
Flag: EXEMPT bug fix
Change-Id: Ibebfa9a2de67c8386eb2c84706245a0046d3ab0a